Adenocarcinomas of the lung, without squamous differentiation or obvious mucus production, from 51 patients were studied. Twenty-eight tumors (55%), when stained with rabbit anti-human surfactant apoprotein antiserum by the peroxidase-antiperoxidase method, demonstrated characteristic nuclear inclusions. Most of these tumors could be identified histochemically by the presence of eosinophilic, PAS-positive nuclear inclusions. In patients with apoprotein immunoreactive tumors there were nine deaths due to tumor (32%) within five years of diagnosis. Eight of the nine deaths occurred in patients whoe tumors exceeded three centimeters in diameter; and an equal number of patients were smokers. The average age of patients with apoprotein positive tumors was 67.3 years, a figure greater than that for patients bearing apoprotein negative tumors (61.4 years). Five of the 23 patients with apoprotein negative tumors died of their neoplasms, in two of the subjects the tumors exceeded three centimeters in diameter. In that no significant difference in survival was observed among patients with apoprotein positive and apoprotein negative tumors, we conclude that subclassification of adenocarcinomas of the lung as apoprotein positive, i.e., type II pneumocytic, or as apoprotein negative types, is of no clinical or prognostic significance. Nonetheless peripheral tumors measuring less than 3 centimeters in size, showing neither squamous differentiation nor obvious mucus production, should be recognized as a prognostically favorable group in comparison with other types of lung carcinomas. (4)